This invention relates to exposure control systems for photographic cameras and more particularly, concerns improvements in such control systems by which the useful range of a camera may be economically extended to provide automatic exposure of high-speed film at low levels of non-transient light without comprising an existing capability for exposing high and low speed films by natural light or by flash.
Light sensitive electronic timing circuits for controlling shutter speeds are extensively used in photographic cameras ranging from the most sophisticated professional varieties to extremely low-cost cameras most commonly used by amateurs. In this latter class of cameras, the least expensive models are capable of use with only one speed of film and equipped with one lens aperture, thus leaving the automatic shutter as the only variable exposure parameter for accommodating variations in scene light levels. In more sophisticated exposure control systems of the type disclosed, for example, in U.S. Pat. No. 3,205,797 issued Sept. 14, 1965 to E. H. Land et al. and in U.S. Reissue Pat. No. 26,627 issued July 15, 1969 (original No. 3,205,803, issued Sept. 14, 1965) to John P. Burgarella et al, both of which are assigned to the assignee of the present invention, the combination of shutter timing circuits is correlated with adjustable lens aperture stops to accommodate widely varying film speeds as well as wide ranges of scene light levels at which the respective films may be exposed.
To provide the amateur photographer with a choice of relatively low-speed (ASA 75) color film, or of relatively high-speed (ASA 3000) black and white film, low-cost cameras are presently available in which a single shutter timing circuit is adapted to either film by means of an aperture plate slidable between two positions so as to provide a relatively small diaphragm stop for the fast black and white film and a maximum relative lens aperture for the slow color film. As a result of the diaphragm stop adjustment for film speed, the light level variation of a scene to be photographed will be accommodated by the automatic shutter within the normal daylight range of illumination for scenes out of doors. Also such cameras may be provided with a flash illumination capability as a result of a switch actuated by the insertion of a flash bulb and operable to alter the light sensitive timing circuit components to provide a maximum flash shutter time of 200 ms.
Now, the speed of the black and white film renders such films suitable for rapid exposure in relatively dim light, however, the exposure parameters available in the above noted cameras are exceeded under such conditions. Hence, it is desirable to provide the latter capability in cameras of the above type while retaining compactness and economy.
Consequently, the objects of the present invention are therefore: the provision of an exposure control system for photographic cameras in which automatic shutter circuit sensitivity to scene illumination is extended to include an additional level of transient light; the provision of such an exposure control system for existing camera constructions with a minimum modification of existing components and at a minimum of expense; and the provision of such an exposure control system which facilitates the indication of camera operating mode to the user of the camera.